Note: Descriptions are shown in the official language in which they were submitted.
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MODEII SUITED FOR WIRELESS
CO~$UNICATION CHANNE~ USE
Background of the Invention
This invention relates to a modem and a modem
signalling protocol especially suited for use with
wireless communication equipment. A described method of
modulation of digital datA is especially suited for use
with conventlonal tws-way FM radios.
The transmission o~ digital data over a radio
link has been accomplished by the use o~ 202 modems which
provide direct audio frequency shift keying (FSg) to the
normal alld1o input of a ~wo-way radio. The use o~ 202
modems in such a system requires no speoial signalling
protocol considerations 6ince the substitution of radios
:~ for the normal wireline communication chann~l does not
disrup~ the normal tone signalling.
Summary o~ th~ Invenkion
It is an~object of the pres2nt invention to
provide a modem signalling protocol compatible with the
needs of conventional data terminal equipment (DTE) where
baseband modulation of conventional two-way radio~ is
used.
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Another object of this invention is to provide a
modem with an improved communication protocol which
allows DTE with diP~erent data rates to communicate with
each other.
An embodiment o~ a modem according to the present
inv~ntion communicates digital data with DTE, demodulates
signals receiv~d from a two-way radio to recover the
digital data, and provides a baseband modulation signal
to the two-way radio which carries the digital data to be
transmitted. The modem overcomes the lack o~ a
communication channel carrier for data carrier d~tection
(DCD). There is generated and transmitted by the radio a
first digital command word representing DCD true prior to
the transmi~sion o~ the digital data. A second cligital
command word is generated and transmitted representing
DCD false at the end of the transmission of the cligital
data. A DCD signal is provided to the DTE controlled
according to the first and ~econd digital command words.
The modam also provide~ control of the two way radio
transmitter in response to a request to ~end (RTS) signal
by the DTE.
~ he modem also accommodates disital data ~rom a
DTE at a lower rate than the predetermined con~tant data
rate o~ the modem. This allows higher speed DTE's to
make use of the higher modem rate without raquiring the
modem rate be adjusted to the slowest data rate
encountered. A buffer stores the digital data from a DTE
running at a slower rate. The modem transmit~ the stored
digital d~ta at the higher predetermined data rate o~ the
~odem, and also transmits predetermined filler data
between the transmission o~ the stored digital data in
order to maintain a constant transmission rate over the
modem.
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Brie~ Descriptlon of the Drawin~s
Figure 1 illustratss a wixeless two-way
communication system according to the present invention.
Figures 2 and 3 are block diagrams illustrating
respectively khe transmitter and receiv~r portions o~ an
embodiment o~ a modem according to the pre~ent invention.
Figures 4 and 5 illustrate a digital ~ormat
utilized for data bytes and command bytes, respectively.
Figure 6 illustrates the digital signalling
protocol utilized to transmit a data carrier detection
(DCD) true command.
Figure 7 illustrates the digital protocol
utilized to send a DCD false command.
Detailed Descri~tlon
Figurs 1 illustrates a wireles communication
system utilizing ~F modems 10 aacording to th~ prssent
invention. The modems aommunicate digital data with data
terminal equip~ent (DTE) 12, provide appropriate
; modulating signal~ 14 to conventional two-way ~,M ra~ios
16, and decode signals 1~ received by the radios. If
~ull duplex radios 16 are utilized, data transmission is
simultan~ously permitted in both directions~ If only
hal~ duplex ra.dios are used, then only hal~ duplex data
trans~ission is av~ilable. The DTE represents any type
oX digital eguipm~nt which transmits or recaives digital
data. In one application of the present invention,
digital infvrmation from a portable optical bar code
reader is transmitted via two-way radio to a central
computer whi.ch maintains lnventory and pricing
infor~ation o~ a variety of products such as might be
~ound in a department store.
In figure 2, the transmitter or modulation
portion of the RP modem 10 i5 shown. Digital data to be
transmitted and a request to send (RTS) signal are
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received from the DTE and clear to send (CTS) is provided
to the DTE. Upon receiving an RTS signal, control encode
20 provldes an output signal 22 to ho:Ld-on circuit 24
which immediately providas a true signal 26 which is
connected to the push to talk (PTT) illpUt of the radio
10. Th~ PTT turns on the transmitter in radio 10 in
preparation for data transmission. Hold-on circuit ~4
may comprise a conventional type o~ fast attack slow
drop-out time delay circuit. This will provide the
desired fast turn on of the transmitter with a delayed
turn off to provide time for the and of data
transmissions. Control encode 20 al60 provides a digital
protocol sequence on line 28 which will be describsd with
regard to figure 6. Control encode 20 may consist of
conventional hardware logic or may be implemented in
software in accordance with the parametrs described
below. After a delay determined by delay circuit 32, CTS
i provided to the DTE in respons~ to the RTS request.
This d~lay prohibits the DTE ~rom beginning data
transmission unt 1 the communication channe~ has been
established.
The DTE data i~ received by buffer 34 which may
comprise a shi~t register and is transmitted by line 38
to data control multiplexer 30 pursuant to a clock input
provided ~y DTE clock 36. The DTE clock rate is set at
the data rate of the DTE. The output of multiplexer 30
is recsived by a conventiona} data scrambler 40 which
provide~ randomization of the transmitted bits to insure
a minimum of low frequency components in the signal
spectrum. Such a scrambler may comprise a conventional
~hift register with feedback. The output of the
scrambler i5 ~urther processed by a non return to zero
inverting (NRZI) encoder 42 which provides a data stream
which i~ not polarity sens~tive. That is, an inversion
o~ the data stream ~etwQen the output of the modem
transmitter and the receiving modem will not prevent
recovery of the original digital data by the receiver
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modem. NRZI coding rules and implementation of NRZI
coding are known.
Low frequsncy boost circuit ~4 provides
additional amplification of low fre~lency portions of the
data spectrum received from NRZI encoder 42 in order to
compensate for the possibla lack of ~,ufficient low
frequency response by a conventional FM radio. The low
fre~uency response of the radio transmitter must extPnd
to approximately 0.001 times the baud rate assuming an 8
bit scrambler is used. The output o~E boost circuit 44 is
filtered by filter 46 before being passed as an analog
baseband output to the FM radio. The purpose of the
filter is to shape the spectral response of the data to
ensure compliance with restrictions on transmitted
frequency spectrum. The low ~requency boo~t circuit 44
may comprise a low pass filter and the ~ilter 46 a
bandpass filter.
The RF modem according to the present invention
provides direct baseband modulation (varying DC lsve}s)
~o the FM radio. This modulation is applied to a direct
input to the FM radio modulator which is available in
conventional FM radios. For example, ~otorola two-way FM
portable radios commonly provide an input referred to as
the "PL" or ~DPL" which effectively couples a modulating
signal through a capacitor to the transmitter's
modulator. The use of this modulation technique produces
- direct Prequency modulation about the center frequency of
the two-way xadio. Since no audio tones or signal
carrier~ ars used, there ~s no means ~or conventional DCD
recognition based upon such carriers. The present
invention provides an alternative means for providing DCD
control which is sxplained below.
Figure 3 illustrates the receiver or demodulating
portion of the RF modem 10 in which bandpass filter 48
receives an analog input signal from the FM radio,
preferably from the discriminator output of the radio.
The ~iltered data provides an input to comparator 50
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which provides outpu~ logic 1 and 0 levels to NRZI
decoder 52. Thi~ decoder i6 the functional co~plement o~
~he NRZI ~ncoder 42 ln ~he modem tran~:mitter ~nd provldes
non return ~o zero d~ta output. NRZI ~ncoder 42 and
decoder 52 cooperate to ellmlnate a potentlal ~ata
recovery problem which would occur ir an unan~icipated
polarlty inversion resulted due to the particul~r FM
r~dlos utiliz~. Th~ output o~ the decoder i~ ac~ed upon
by descrambler ~4 which per~orms t~e inver6e ~unc~ion o~
scrambler ~0 ln order to raconstitu~e and recover the
original data. The receiYed data i~ an inpu~ to d2ta
con~rol demultiplexer 56 whlch routes data byt~6 to f
buffer 58 and nondata bytes in~ludlng co~mand and f:Lller
byt~s (diseus6ed below~ to control decode 60. The
de~ultiplexer 56 ~ay consist o~ conventional logic gates
or ~ay be imple~ented in so~tware ~o provlde the runction
according ts protocol a~ described below. A phase locXed
loop 51 rec~ives the filtered data output ~ro~ comparator
50 and provides a clock output to ~lement~ 52~ 54 and 56.
Control decod~ S0 ~unctlon~ to recogni2e
pr~d~term~n~d co~mand byt~ and provide fun~tional
outpl~t~ c~rr~sponding to thQ ~un~tio~ corre~pondlng to
sach ~ommand byte. ~he ~xemplary com~an~ byte repre3en~s
the DCD co~and which ~rovid~ DCD tru~ ~ignal on
o~pu~ line 62 ~o ~he receiYing DTE. The da~a bytes
6tored in bu~fer 58 which may consis~ o~ a ~hift register
- or ~emory allocation are transmitted to tha receiving DTE
on ths RX data line 64 at tha rQceiving DTE rate as
datermin~d by DTE clock 66.
The total channel response time a~ meaeured from
the output of NRZI encoder 42 in the ~odem tranem~ttsr to
the input to comparator S0 in the modem receiver ehould
be egual to or le~ than the number o~ bits per baud
divid~d by the bit rata. Pra~arably, ~his ~ime response
35 is dom~ nat d by the characteri~tics of ~ilters 46 and 48.
Filter 46 ha~ an additional constraint in tha~ it ~ust
provide ~u~ficient ~iltering to meet controlled emi~sion
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standards. For example, a modem data rate of 4800 bits
per second and a 1 bit per baud format would require a
response time of approximately 208.3 microsecondsO
The protocol used with the R~ modems according to
the presQnt invention includes DCD information and allows
different DTE to communicate predetermined length
mes~ages at different data rates. The protocol
contemplates a~ynchronous modem data.
Figures 4 and 5 illuqtrate the data and command
byte formats, respectively. Each data byte i~ defined by
a start bit - 0, eight data bits, and a stop bit ~
Each command byte is defined by a start bit - 0, eight
bits, and a stop bit - 0. The eight available bits in
the command byte provides for 256 possible system
commands. In the illustrated system, only three commands
are defined: DCD true, DCD false and a stuf~ (filler)
byte. It will be apparent that command bytes are defined
by a 0 stop bit.
Referring to figure 6, a three byte pattern
generated by con~rol encode 20 is repeated for the time
interval defined by delay circuit 32 which corresponds to
the delay between RTS and CTS. The three byte paktern
consists of character one, character two, and FF(H) (FF
in hexidecimal). Control decode 60 in the RF modem
r~ceiver provides a DCD true output on line 62 to the
receiving DTE following detection of the thre2 byte
sequence and detection of the first stuff byte. A ~tuff
byte is a command byte which has no corr~sponding command
function. It is used as filler information which is
utilized to permit different DTE data rates as will be
explained below. A stuff byte i9 generated at the end of
the three byte pattern following CTS changing from false
to krue.
In figure 7, it will be seen that two end of data
(DCD false) command bytes are sent at the end of data
transmission in order to ensure that control decode 60
recognizes the command and turns DCD line 62 from true to
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fal6e. The two end of data command bytes are separated
by an FF(H) byte to enhance character synchronization and
hence maximize the probability that the second end of
data command byte wlll be correctly recogniæed. The
origination of the end o~ data byte by control encode 20
in response to RTS changing from true to false releases
hold-on circuit 24 allowing the two-way radio transmitter
control by PTT to be turned off after a delay associated
with hold-on circuit 24. This delay is sufficient to
allow completion of the end of data sequence befora the
transmitter turns o~f.
The ~F modems are capable o~ operatlng in a
system with DTE having di~ferent data rates. Stuf~ bytes
are inserted in the transmitt~d data stream to fill idle
time over the RF communications link in order to maintain
clock and data synchronization. For example, this allows
a DTE operating at 4800 bps to communicate with a DTE
operating at 1200 bps over a modem communication channel
operating at 4800 bps. If the faster DT~ is the
originating (transmitting) DTE, the received data will
have to be stored at the receiving modem and sent to the
slower DTE at its rate. This is accomplished by the
buffer 58 in the RF modem receiver which operates under
the control of the receiving DTE clock. The length o~
messages which can be communicated at a higher rate to
the slower DTE is determined by the length of buffer 58.
When the slower DTE sends data, the intervening time
frames between data is illed with stuff bytes generated
by control encode 20 to maintain clock and data
synchronization. In the receiver of the RF modem, the
data control demultiplexer 56 sends the data bytes to
buffer 58 and sends the stu~ bytes to control decode ~Q.
Since ths stuff bytas do not correspond with
predetermined command bytes, the stuff bytes are ignored.
This technique allows the RF modem and RF communications
link to operate at the highest data rate available and
yet accommodate communications with DTE having a slower
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or different data rates. In a system with different
speed DTE, high speed DTE can communicate at lower
throughput speeds with slower DTE while being able to
communicate at fu11 throughput sp~ed with other high
speed DTE.
Although an emhodiment of the present invention
has been described and i11ustrated in the drawings, the
scope of the invention is defined by the clalms appended
hereto.
